1. Field of the Invention
The present invention relates to a polar amplification stage which is suitable for, but not limited to, a transmitter. The invention is particularly but not exclusively concerned with an amplification stage in which an envelope tracking (ET) modulator is utilised to provide a power supply to an RF amplifier.
2. Description of the Related Art
Polar amplification stages, also known as polar modulators, are well-known in the art, and comprise an arrangement in which separate magnitude and main signal paths are utilised in order to amplify a signal for transmission. Polar amplification stages are typically used in transmitters, such as transmitters for RF (radio frequency) applications. A mobile communication system RF implementation may use a polar amplification stage.
Known polar amplifiers utilise, for example, envelope tracking (ET) techniques or envelope elimination and restoration (EER) techniques. In an envelope tracking amplification stage a modulated power supply is generated for an amplifier in dependence on the envelope (magnitude) of the input signal to be amplified by the amplifier, and the input signal to be amplified is provided as the input of the amplifier.
It is an objective in such amplification stages to minimise the distortion in the transmitted signal. It is known that one cause of distortion is a result of the total delay suffered by the main signal (in a main signal path) and the total delay suffered by the magnitude signal (in a magnitude signal path) not being equal to each other. In order to meet system specification requirements, it is necessary to align the signal in the main signal path and the magnitude signal path to ensure that any timing misalignment between those signals falls within a permitted range. Any misalignment of the timing signals in the paths results in distortion of the transmitted signal, and can reduce transmitter efficiency.
With reference to FIG. 1 there is illustrated components of an exemplary known RF amplification architecture in which an envelope tracking (ET) modulator is used to provide a power supply to a radio frequency (RF) power amplifier.
As illustrated in FIG. 1, an RF power amplifier 102 receives an RF input signal to be amplified on an input line 136, and receives a modulated power supply voltage Vsupply on line 138. The RF power amplifier 102 generates an RF output signal on line 140. An example implementation of such an RF power amplifier is in mobile communication systems, with the RF output on line 140 connected to the front end of radio transmission equipment.
A signal generation block 122 receives a baseband signal (not shown) to be amplified by the amplifier 102. The signal generation block generates a signal on line 125a representing the envelope of the input signal to be amplified. The signal generation block 122 additionally generates I and Q components of the input signal to be amplified on lines 125b and 125c. 
The generation of the envelope signal and the I and Q components of the baseband input signal is known to one skilled in the art. Various techniques for the generation of such signals may be implemented.
As illustrated in FIG. 1, the envelope signal on line 125a representing the envelope of the input signal to be amplified is converted by a digital-to-analogue converter 126a into an analogue signal, filtered by an optional envelope filter 128a, and then provided as an input to an ET modulator 108. The ET modulator 108 may be implemented using a variety of techniques. For example, the ET modulator 108 may incorporate a switched mode power supply for selecting between one of a plurality of supply voltages in dependence on the magnitude of the envelope signal, and a correction or adjustment stage for adjusting the selected supply voltage in dependence on an error determined between the selected supply voltage and a reference signal based on the envelope signal. An exemplary envelope tracking modulator is described in U.S. Pat. No. 7,482,869. The output of the ET modulator 108 forms an input to an output filter 106, and a modulated supply voltage is then provided through a supply feed 104 to provide the supply voltage on line 138.
The baseband I and Q components of the input signal on lines 125b and 125c are converted into analogue signals via respective digital-to-analogue converters 126b and 126c, and optionally filtered through respective I and Q filters 128b and 128c. The filtered I and Q components of the input signal are provided as inputs to a vector modulator, illustrated as respective multipliers 130a and 130b and a combiner 132. The combined output of the combiner 132 forms an input to a variable gain amplifier 134, the output of which forms an input to an optional inter-stage surface acoustic wave (SAW) filter 112. The output of the filter 112 provides the RF input signal to be amplified on input line 136 to the RF power amplifier 102.
As known in the art, the envelope path which the envelope signal follows from the digital-to-analogue converter 126a to generation of the supply voltage on line 138 to the power amplifier 102 suffers from delays which vary on a unit-by-unit basis within a production tolerance. As also known in the art the input or RF path which the baseband signal follows from the digital-to-analogue converters 126b and 126c to generation of the RF input signal to be amplified on line 136 suffers from delays.
In general, such delays need to be controlled so as to ensure that they fall within certain tolerances, usually smaller than the production tolerances, to ensure maximum operating efficiency of the power amplifier and to ensure certain spectral emissions requirements are met (such as a minimum distortion of the amplified output signal).
In the envelope path delays may be introduced by several stages, such as the filter 128a, the output filter 106, and the supply feed 104. In addition delays may arise in the ET modulator 108 itself.
In the input or RF path delays may also be introduced by several stages, such as the respective I and Q filters 128b and 128c, and in the inter-stage SAW filter 112.
It is an aim of the present invention to provide an improved technique for a polar modulator in which distortion in an amplified signal is reduced by controlling the relative delay between the main signal path and the magnitude signal path.
It is also an aim of the present invention to provide an improved technique for an envelope tracking power supply for an RF amplifier, in which distortion in an amplified signal is reduced by controlling the relative delay between in the RF signal path and the envelope signal path.